Many widely-used image sensors include global-shutter pixels because of their high efficiency and lack of a blur as compared to rolling-shutter pixels. Global shutter pixels typically include a storage node, separate from the photodiode region, where charge generated during a previous integration frame can be stored and subsequently read out. All of the pixels in a global shutter image sensor typically transfer charge generated in their respective photodiodes to their respective storage nodes ‘globally,’ which eliminates the blur caused by the row-by-row exposure and readout in rolling shutter pixels. The global shutter storage nodes can be read out when convenient, such as while the photodiode is integrating charge for a subsequent frame.
The storage node in global shutter pixels is usually located on the same surface of a semiconductor wafer as the photodiode region, and thus typically needs to be shielded in order to maintain the integrity of the charge stored in the storage node. Also, positioning the storage node on the same surface of a semiconductor wafer as the photodiode reduces the amount of surface area of the photodiode that can be exposed to light, and hence reduces the sensitivity of the pixel.